Radiowave watches have a function to correct time by receiving magnetic field components of electromagnetic waves containing time information. Keyless entry systems enable owners of units transmitting and receiving particular electromagnetic waves to open and close keys of cars, houses, etc. without contact. RFID (radio frequency identification) systems send and receive information to and from tags with particular electromagnetic waves. For example, when RFID tags having destination information, etc. of buses, etc. are attached to buses, and when RFID tags having timetable information are embedded in timetable boards of bus stops, etc., users can recognize various types of transportation information without contact.
The keyless entry systems, etc. use radiowaves having frequencies of 40-200 kHz (several kilometers of wavelength). For example, two types of radiowaves of 40 kHz and 60 kHz are used in Japan, and mainly frequencies of 100 kHz or less are used overseas. For systems receiving electric field components of such long-wavelength radiowaves, antennas over several hundreds of meters are needed, not suitable for small radiowave wristwatches, small keyless entry systems and small RFID systems. Accordingly, it is preferable to use systems for receiving magnetic field components of long-wavelength radiowaves with magnetic sensor-type antennas comprising coils wound around magnetic cores.
As shown in the equivalent circuit of FIG. 14, when a magnetic field component of an electromagnetic wave input to an antenna flows through a magnetic core, voltage V induced in a coil L wound around the magnetic core resonates by a parallel resonance circuit of a coil L and a capacitor C, so that resonance current flows in the coil L at voltage Q times, wherein Q is a characteristic value of the resonance circuit. Because the antenna is disposed mostly in a metal casing, magnetic flux from the magnetic core ends flows through an adjacent metal casing, losing magnetic energy as eddy current loss. Accordingly, antennas used in radiowave wristwatches, etc. should be small, and there should be little magnetic flux leakage to reduce the eddy current loss.
In addition, receiving antennas for wristwatches, keyless entry systems, RFID systems, etc., whose magnetic core directions are changing every moment, are required to be omnidirectional, namely to have high receiving sensitivity in any directions of XYZ axes. As a technology for being omnidirectional, for example, JP 2002-217635 A discloses an antenna apparatus comprising coils perpendicularly wound around pluralities of rod-shaped, magnetic cores and connected in series. JP 2004-229144 A discloses a surface-mounted antenna comprising coils wound around pluralities of cross-shaped, magnetic cores projecting from a center base member. However, because of pluralities of rod-shaped, magnetic cores, these antennas are not suitable for small radiowave wristwatches, etc. with little space for antennas.
JP 2001-320223 A discloses a radiowave watch comprising an omnidirectional antenna comprising pluralities of coils wound around an integral, planar, ring-shaped, magnetic core in different directions. However, winding coils around the integral, ring-shaped, magnetic core needs time-consuming work.
JP 2000-105285 A discloses a portable radiowave watch comprising a housing, a watch module disposed at a center of the housing, an external operation means for the module, a groove surrounding the module in the housing, and an antenna received in the groove. The antenna is constituted by a C-type magnetic core and a coil wound around the magnetic core. However, the antenna of this structure has strong directivity.
JP 2005-102023 A discloses a receiving antenna structure disposed in a metal casing, which comprises a main magnetic path member comprising coils wound around a magnetic core, a sub-magnetic path member comprising a coil-free magnetic core, and a gap in a closed magnetic path along the magnetic core, thereby preventing magnetic flux from leaking outside during resonance. However, this antenna also has strong directivity.